The present invention relates in general to laser beam scanning systems, and is particularly directed to a acoustic impedance transformer, which is interposed between a relatively large sized, electrically driven piezo-electric transducer and an acousto-optic medium. The acoustic impedance transformer is configured to effectively match the impedance of the acoustic transducer with that of the acoustic wave propagation medium of the acousto-optic waveguide.
FIG. 1 diagrammatically illustrates the configuration of a guided acoustic travelling wave lens devicexe2x80x94one that employs a relatively narrowly dimensioned traveling wave channelxe2x80x94as comprising a laser 10, the optical beam output 11 of which is focussed by a cylindrical lens arrangement 12 and deflected by a mirror 13 onto an acousto-optic beam deflector 14, to which an RF input signal is applied. The acousto-optically modulated beam is then reimaged by a further sphericalxe2x80x94cylindrical lens arrangement 15 onto a traveling lens cell 16, that contains a traveling wave lens transport medium 17 and a traveling wave lens launching transducer 18. The scanned beam is then imaged onto an image collection medium, such as a photographic film 19.
In a number of applications, the acousto-optic waveguide may be configured as a reduced height, guided acoustic travelling wave lens (ATWL) waveguide device, such as that diagrammatically illustrated at 30 in FIG. 2. In this type of acoustic wave guide architecture, a first end 32 of the waveguide has an acoustic wave input aperture 34 (to which an acoustic wave-launching piezo-electric transducer is coupled), at an input end of a relatively narrow (fluid-containing) channel 36, having a cross-section of width w and height h, where w greater than  greater than h.
For a non-limiting illustration of examples of. documentation describing such guided acoustic traveling wave lens devices, attention may be directed to an article entitled: xe2x80x9cOptical Beam Deflection Using Acoustic-Traveling-Wave Technology,xe2x80x9d by R. H. Johnson et al, presented at the SPIE Symposium On Optical, Electro-Optical, Laser and Photographic Technology, August 1976, FIG. 6 of which corresponds to FIG. 1, above, an article entitled: xe2x80x9cGuided acoustic traveling wave lens for high-speed optical scanners,xe2x80x9d by S. K. Yao et al, Applied Optics, Vol. 18, pp 446-453, February 1979, and the U.S. Pat. No. 3,676,592 to Foster.
In a reduced height guided wave device, because the acoustic transmission properties of the acoustic propagation medium (fluid) within the waveguide channel 36 are considerably different from those of the transducer being used to launch the acoustic wave into the waveguide, there is a substantial acoustic impedance mismatch between the transducer and the waveguide. Indeed, the acoustic impedance of the waveguide may be on the order of twenty or more times that of the transducer.
In such a circumstance, in order to provide significant energy coupling from the transducer to the waveguide""s acoustic propagation medium (e.g. water), the transducer must be allowed to resonate to very large internal power. This causes two problems. First, the acoustic transducer is prone to failure, as the result of the very substantial acoustic stresses required. Second, the bandwidth is limited.
In accordance with the present invention, this electrical and, acoustic impedance mismatch problem is successfully obviated by means of an acoustic impedance-matching transformer that is inserted between the acousto-optic medium and a piezo-electric transducer. The acoustic impedance-matching transformer is configured to effectively match the acoustic impedance of the acoustic transducer with that of the acousto-optic medium.
For this purpose, the acoustic impedance transformer is structured as a combination of steps and tapers, in the form of a cascaded series of acoustic propagation elements of successively decreasing acoustic impedance. At each interface between adjacent elements, the abutting surfaces or the elements are physically configured to provide an effectively acoustic impedance match between the elements, and thereby an efficient coupling of the acoustic energy from an element of relatively higher acoustic impedance material to an element of relatively lower acoustic impedance material. Over the length of the transformer, this sequential xe2x80x98steppingxe2x80x99 of the acoustic impedance and configurations of the abutting surfaces of successively adjacent elements operates to effectively match the acoustic impedance of the piezo-electric transducer to that of the liquid acoustic traveling wave lens.
The first acoustic wave propagation element of the transformer comprises a relatively dense, acoustic energy transmission material, such as a metal (e.g., aluminum) block. This first element has an acoustic impedance that differs (or is stepped down) from that of the piezo-electric transducer in a direction toward the relatively small acoustic impedance of the acoustic propagation medium (water) within the ATWL waveguide. In order to compensate for this difference in the acoustic impedances of the materials of the transducer and the aluminum block, the dimensions of the two materials in the direction of the step must be smaller than the acoustic wavelength in the respective material. To this end, the spatial dimensions of the surface of the block are larger than those of the abutting wave-launching surface of the transducer, and by an amount proportional to the inverse ratio of their respective acoustic impedances, whereby the resulting acoustic-coupling interface provides broadband efficient coupling of acoustic energy therebetween.
Although such dimensioning of the engaging faces of the piezo electric transducer and the aluminum block ensures efficient coupling of acoustic energy from the transducer into the aluminum block, the cross-section of the aluminum block at its acoustic energy receiving face adjoining the piezo-electric transducer is still larger than that of the acoustic wave coupling aperture of the waveguide channel. Also, the acoustic impedance of the aluminum block is considerably larger than that of the waveguide channel.
These differences are compensated by tapering the aluminum block toward the aperture of the waveguide channel, so as to focus the acoustic energy to a high acoustic energy density value at a reduced cross-section end face adjacent to the acoustic input aperture to the waveguide, and by providing a second transformer element between the reduced cross-section end face of the tapered aluminum block and the acoustic input aperture to the waveguide. The extent to which the first transformer block is tapered is determined in accordance with the difference between the acoustic impedance of the aluminum block and that of the second acoustic wave propagation element, such as a section of plexiglass or the like, so as to provide a compensating abutting surface area mismatch therebetween, as in the case of the piezo-electric transducer and the input end of the tapered aluminum block.
The plexiglass section has a cross-section corresponding to that of the acoustic input aperture of the waveguide. Since plexiglass has an acoustic impedance that is approximately twice that of water, it is configured as a quarter-wave plate section, so as to provide an acoustic impedance match between its interface with the reduced cross-section end of the tapered aluminum block and the waveguide.
The combined effect of the increased acoustic power density at the exit end of the aluminum block, the increased cross-sectional area of the plexiglass quarter-wave plate, and the ratios among the acoustic impedance parameters of the mutually adjoining aluminum block, quarter-wave plate and waveguide channel thereby provides an efficient, broadband coupling into the waveguide channel of acoustic energy originally launched from the piezo-electric transducer.